AU3371801A - Tyre tread comprising an emulsion styrene/butadiene copolymer - Google Patents

Description

Title: Tyre tread comprising a styrene/butadiene emulsion copolymer. The present invention relates to a tyre tread comprising a crosslinkable rubber composition which comprises at least one emulsion copolymer of styrene and butadiene and 5 to a process for improving the wear resistance of such a tread. The invention applies to a tread comprising a majority proportion of carbon black as reinforcing filler. Rubber compositions for tyre treads may, in known manner, comprise copolymers of styrene and butadiene (also referred to as SBR in the remainder of the present description), alone or associated with other elastomers depending upon the desired properties. 10 SBRs are most frequently prepared by an emulsion process, i.e. by associating an emulsifier with the monomers in an aqueous medium. This emulsifier fulfils three main functions. It is thus provided: - to produce a stable and well dispersed emulsion of the monomers, - to solubilise the monomers within micelles, where said monomers will be more 15 accessible to free radicals, and - to prevent precipitation of the copolymer formed. The emulsifiers used primarily comprise fatty acid soaps, such as soaps of, for example, capric, lauric, myristic, palmitic, stearic or oleic acid, or alternatively soaps of resin acids (also known as resin soaps or rosin soaps), such as soaps of acids of the abietic or 20 hydroabietic type, for example of tetrahydroabietic acid. Synthetic emulsifiers, such as aryl sulphate, sodium lauryl sulphonate or cumene peroxide, may also be used. There are two major types of emulsion copolymerisation processes for styrene and butadiene, one of which, the high temperature process (performed at a temperature of the 25 order of 50'C) is suitable for preparing highly branched SBRs, while the other, the low temperature process (performed at a temperature which may range from 1 5C to 40'C), allows the production of more linear SBRs. A detailed description of the effectiveness of several emulsifiers usable in said high temperature process (as a function of the amounts of said emulsifiers) may be found by 30 referring, for example, to two articles by C. W. Carr, 1. M. Kolthoff, E. J. Meehan, University 2 of Minnesota, Minneapolis, Minnesota, which were published in Journal of Polymer Science, 1950, Vol. V, no. 2, pp. 201-206, and 1951, Vol. VI, no. 1, pp. 73-81. With regard to comparative examples of the performance of said low temperature process, reference may be made, for example, to the article "Industrial and Engineering 5 Chemistry, 1948, Vol. 40, no. 5, pp. 932-937, E. J. Vandenberg, G. E. Hulse, Hercules Powder Company, Wilmington, Delaware" and the article "Industrial and Engineering Chemistry, 1954, Vol. 46, no. 5, pp. 1065-1073, J. R. Miller, H. E. Diem, B. F. Goodrich Chemical Co., Akron, Ohio". to In general, it will be noted that the higher is the concentration of emulsifier in the monomer mixture, the higher is the resultant rate of copolymerisation, this phenomenon continuing until a monomer conversion approaching that relating to completion of the reaction is reached. Furthermore, this increase in emulsifier concentration gives rise to an SBR which [5 imparts still further improved "uncured adhesion" (i.e. the ability to adhere in the unvulcanised state to other unvulcanised rubber compositions) to the rubber composition into which it is incorporated. It will also be noted that the presence of an excess of emulsifier, i.e. typically in a concentration of greater than 8 phr (parts by weight per 100 parts of elastomer), makes it 20 difficult to extract the copolymer without degrading the macrostructure thereof and to recover unreacted monomers, due to the presence of foam formed by the excess emulsifier in the aqueous phase. Furthermore, this excess of emulsifier also imparts mediocre physical properties to a vulcanised rubber composition comprising the SBR prepared in this manner. 25 Conversely, a low emulsifier concentration in said monomer mixture, typically of below 4 phr, results in a considerably reduced rate of polymerisation. Japanese patent document JP-A-82/53 544 may be cited in this regard, said document disclosing the use of SBRs prepared in an emulsion with a reduced emulsifier content (less than or equal to 3 phr) for reducing the rolling resistance of tyre treads which contain them. 30 It is for this reason that commercially available emulsion SBRs are characterised by an emulsifier concentration which is conventionally between 4 and 8 phr. The person skilled in the art knows that emulsion SBRs are well suited to use in the 5 unvulcanised state. The SBRs may also be prepared in solution by anionic polymerisation in a hydrocarbon solvent, the reaction being performed by means of a lithiated initiator. The SBRs prepared in this manner in particular exhibit physical properties in the vulcanised state and 10 resistance to wear which are satisfactory. One major disadvantage of conventional emulsion SBRs is the elevated hysteresis exhibited by tyre tread compositions comprising these SBRs in comparison with those comprising solution SBRs. 15 The object of the present invention is to provide a tyre tread having improved wear resistance which comprises a crosslinkable rubber composition comprising, on the one hand, at least one elastomeric emulsion copolymer of styrene and butadiene and, on the other, a reinforcing filler comprising carbon black in a majority proportion (i.e. in a mass fraction of 20 greater than 50%), in such a manner that said carbon black is present in said composition in a quantity of greater than or equal to 55 phr. The applicant has surprisingly discovered that an emulsion SBR prepared in such a manner that it comprises an emulsifier content substantially varying from 1 to 3.5 phr may 25 advantageously be used in a crosslinkable rubber composition comprising a reinforcing filler as defined above in order considerably to improve the wear resistance of a tyre tread comprising said composition in comparison with that of a tread including a conventional SBR, likewise prepared in emulsion, without impairing and possibly even improving other physical properties in the vulcanised state, in particular hysteresis properties. 30 4 It may be noted that the copolymers of styrene and butadiene which may be used in the present invention may be prepared using a high temperature process or a low temperature process. 5 Preferably, it will be noted that said carbon black is present in said composition in a quantity of greater than of equal 60 phr and, still more preferably, in a quantity ranging from 70 phr to 100 phr. Suitable carbon blacks are any carbon blacks, in particular the blacks of the type HAF, ISAF and SAF, which are conventionally used in tyres, and particularly in tyre treads. As non 10 limitative examples of such blacks, mention may be made of the blacks N 115, N 134, N234, N339, N347, N358, N375. The carbon black may also be used as a blend (mixture) with a reinforcing white filler. In the present application, "reinforcing white filler" is understood to mean a "white" filler (i.e. an inorganic filler, particularly a mineral filler), sometimes also called "clear" filler, 15 which is capable, on its own, without any other means than an intermediate coupling system, of reinforcing a rubber composition intended for the manufacture of tyres, in other words which is capable of replacing a conventional filler of tyre-grade carbon black in its reinforcement function. Preferably, the entirety or at the very least a majority proportion of the reinforcing 20 white filler is silica (SiO2). The silica used may be any reinforcing silica known to the person skilled in the art, in particular preferred highly dispersible precipitated silicas, such as silica Perkasil KS 430 from Akzo, silica BV 3380 from Degussa, silicas Zeosil 1165 MP and 1115 MP from Rhodia, silica Hi-Sil 2000 from PPG, silicas Zeopol 8741 or 8745 from Huber, and treated precipitated silicas such as, for example, the aluminium-"doped" silicas described in 25 application EP-A-0 735 088. As reinforcing white filler, there may also be used, in non-limitative manner, - aluminas (of formula A1 2 0 3 ), such as aluminas of high dispersibility which are described in European Patent Specification EP-A-810 258, or alternatively - aluminium hydroxides, such as those described in International Patent Specification 30 WO-A-99/28376.

Black/silica blends or blacks partially or entirely covered with silica are suitable to form the reinforcing filler according to the invention. Also suitable are carbon blacks surface modified by silica, such as, although this is not limiting, the fillers described in European Patent Specification EP-A-711 805 and the fillers sold by CABOT under the name "CRX 5 2000", which are described in International Patent Specification WO-A-96/37547. In the event that the reinforcing filler consists of carbon black and a reinforcing white filler, the mass fraction of this latter filler in said reinforcing filler is preferably selected to be less than or equal to 30%. 10 The tread composition according to the invention furthermore conventionally comprises, when said reinforcing filler comprises a reinforcing white filler, a reinforcing white filler/elastomeric matrix bonding agent (also referred to as coupling agent), the function of which is to ensure sufficient chemical and/or physical bonding (or coupling) between said white filler and the matrix, while facilitating the dispersion of this white filler within said 15 matrix. The coupling agent optionally used in the rubber compositions according to the invention is advantageously a polysulphurised alkoxysilane, such as a so-called "symmetrical" polysulphurised alkoxysilane. As a particularly preferred example, bis(triethoxysilylpropyl) tetrasulphide, or TESPT, 20 of the formula [(C 2

H

5 0) 3 Si(CH 2

)

3

S

2

]

2 , is used, which is sold, for example, by Degussa under the name "Si69" (or X50S when it is supported to 50% by weight on carbon black), or under the name "Si75" (disulphide) or alternatively by Witco under the name "Silquest A1289". In the rubber compositions according to the invention, the content of bonding agent 25 may be within a range of 0.5 to 15% relative to the weight of reinforcing white filler. The tyre tread compositions according to the invention contain, apart from the elastomeric matrix, the reinforcing filler and optionally one or more reinforcing white filler/elastomer bonding agent(s), all or part of the other constituents and additives usually 30 used in rubber mixtures, such as plasticisers, pigments, antioxidants, antiozone waxes, a vulcanisation system based either on sulphur and/or peroxide and/or bismaleimides, 6 vulcanisation accelerators, extender oils, optionally one or more agents for coating the reinforcing white filler, such as alkoxysilanes, polyols, amines etc.. It will be noted that the tread composition according to the invention may comprise a 5 blend, on the one hand, of one or more emulsion SBRs in a total mass fraction ranging from 50 to 100%, each SBR comprising an emulsifier in said amount of 1 to 3.5 phr, and, on the other hand, one or more essentially unsaturated diene elastomers in a total mass fraction ranging from 50 to 0%. "Diene" elastomer or rubber is understood to mean, in known manner, an elastomer 10 resulting at least in part (i.e. a homopolymer or a copolymer) from diene monomers (monomers bearing two double carbon-carbon bonds, whether conjugated or not). In general, "essentially unsaturated" diene elastomer is understood here to mean a diene elastomer resulting at least in part from conjugated diene monomers, having a content of members or units of diene origin (conjugated dienes) which is greater than 15% (mol%). 15 Within the category of "essentially unsaturated" diene elastomers, "highly unsaturated" diene elastomer is understood to mean in particular a diene elastomer having a content of units of diene origin (conjugated dienes) which is greater than 50%, such as: - any homopolymer obtained by polymerisation of a conjugated diene monomer having 4 to 12 carbon atoms; 20 - any copolymer obtained by copolymerisation of one or more dienes conjugated together or with one or more vinylaromatic compounds having 8 to 20 carbon atoms. Suitable conjugated dienes are, in particular, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C1 to C5 alkyl)-1,3-butadienes such as, for instance, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3 -ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3 25 butadiene, an aryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene. Suitable vinylaromatic compounds are, for example, styrene, ortho-, meta- and para methylstyrene, the commercial mixture "vinyltoluene", para-tert.-butylstyrene, methoxystyrenes, chlorostyrenes, vinylmesitylene, divinylbenzene and vinylnaphthalene. The copolymers may contain between 99% and 20% by weight of units resulting from 30 diene monomers and between 1% and 80% by weight of units resulting from vinylaromatic monomers. The elastomers may have any microstructure, which is a function of the 7 polymerisation conditions used, in particular of the presence or absence of a modifying and/or randomising agent and the quantities of modifying and/or randomising agent used. The elastomers may for example be block, random, sequenced or microsequenced elastomers, and may be prepared in a dispersion or in solution; they may be coupled and/or starred or 5 alternatively functionalised with a coupling and/or starring or functionalising agent. Polybutadienes are preferably suitable, and in particular those having a content of 1,2 units of between 4% and 80%, or those having a cis-1,4 content of more than 80%, polyisoprenes, butadiene-styrene copolymers, and in particular those having a styrene content of between 5% and 50% by weight and, more particularly, between 20% and 40%, a content 10 of 1,2-bonds of the butadiene part of between 4% and 65%, and a content of trans-1,4 bonds of between 20% and 80%, butadiene-isoprene copolymers and in particular those having an isoprene content of between 5% and 90% by weight and a glass transition temperature (Tg) of between -40'C and -80'C, isoprene-styrene copolymers and in particular those having a styrene content of between 5% and 50% by weight and a Tg of between -25 0 C and -50'C. 15 In the case of butadiene-styrene-isoprene copolymers, those which are suitable are in particular those having a styrene content of between 5% and 50% by weight and, more particularly, between 10% and 40%, an isoprene content of between 15% and 60% by weight, and more particularly between 20% and 50%, a butadiene content of between 5% and 50% by weight, and more particularly between 20% and 40%, a content of 1,2-units of the butadiene 20 part of between 4% and 85%, a content of trans-1,4 units of the butadiene part of between 6% and 80%, a content of 1,2- plus 3,4-units of the isoprene part of between 5% and 70%, and a content of trans-1,4 units of the isoprene part of between 10% and 50%, and more generally any butadiene-styrene-isoprene copolymer having a Tg of between -20*C and -70'C. Particularly preferably, the diene elastomer of the composition according to the 25 invention is selected from the group of highly unsaturated diene elastomers which consists of polybutadienes (BR), polyisoprenes (IR) or butadiene-styrene copolymers (SBR), butadiene isoprene copolymers (BIR), isoprene-styrene copolymers (SIR), butadiene-styrene-isoprene copolymers (SBIR), or a mixture of two or more of these compounds. 30 Preferably, a tyre tread according to the invention is such that said or each copolymer comprises said emulsifier in a quantity substantially varying from I to 2 phr.

8 According to another feature of the invention, said emulsifier comprises at least one resin acid and/or at least one fatty acid, in particular oleic acid. According to another feature of the invention, said or each copolymer exhibits a content of trans linkages which is greater than or equal to 70% and a content of styrene 5 linkages substantially varying from 20% to 45%. Furthermore, the number average molecular weight of the said or of each copolymer varies substantially from 110,000 g/mol to 140,000 g/mol. A tyre according to the invention comprises a tread as defined above. 10 The aforementioned features of the present invention, as well as others, will be better understood on reading the following description of several examples of embodiment of the invention, which are given by way of illustration and not of limitation, in comparison with "control" examples illustrating the prior art. In these examples, the properties of the rubber compositions are evaluated as follows: 15 - Mooney viscosity ML(1+4) at 100 0 C: measured in accordance with ASTM:D-1646, hereinafter abbreviated to ML; - modulus of elongation at 100% (M100): measurements taken in accordance with Standard ISO 37, - Shore A hardness: measurements made in accordance with Standard DIN 53505, 20 - dynamic shear properties (G*): measurements as a function of the deformation, performed at 10 Hertz with a peak-to peak deformation from 0.15% to 50%. Hysteresis is expressed by the measurement of tan delta at 7% deformation and at 40'C in accordance with Standard ASTM D2231-71 (reapproved in 1977). 25 I. Examples of elastomers intended for use in a tread according to the invention, in comparison with "control" elastomers: In these examples, testing was performed on: 5 - two elastomers according to the invention, E-SBR A and E-SBR B, each consisting of an emulsion copolymer of styrene and butadiene prepared in a manner known per se and respectively comprising emulsifier contents of 1.7 phr and 1.2 phr, and - two "control" elastomers, E-SBR C and E-SBR D (sold by BAYER under the names "KRYNOL 1712" and "KRYNOL 1721" respectively), each consisting of an emulsion 10 copolymer of styrene and butadiene and respectively comprising emulsifier contents of 5.7 phr and 4.5 phr. Table 1 below summarises the essential characteristics of each of the four elastomers tested with regard to microstructure, properties, formulation and macrostructure. Microstructure was determined in accordance with Standard ISO 6287. 15 Emulsifier contents were determined in accordance with Standard ISO 1407 (for the quantity of acetone extract) and in accordance with Standard ASTM D297 (for non saponifiable content). Furthermore, the quantities of fatty acids and of fatty and resin acid soaps were determined in accordance with Standard ISO 7781. 20 lu Table I E-SBR A E-SBR B E-SBR C E-SBR D MICROSTRUCTURE 1,2 linkages 14.8 13.6 14.9 14.2 (in %) 1,4 linkages 12.4 12.8 13.0 14.2 (in %) Trans linkages 72.8 73.6 72.1 71.6 (in %) Styrene linkages 23.1% 40.9 23.9 38.3 (%) PROPERTIES & CONSTITUENTS Tg -51 -31 -53 -36 Mooney viscosity 64 50 46 54.5 ML(1+4) Density 0.942 0.969 0.947 0.967 Emulsifier (phr) 1.7 1.2 5.7 4.5 Fatty acids 14.8 15.4 169 166 (mEg/kg) Fatty & resin 14.1 11.3 18.3 17.5 acid soaps (mEg/kg) Oil content (phr) 38.5 38.1 38.1 37.9 MACROSTRUCTURE Mn (g/mol) 129,648 113,197 119,397 135,210 Mw (g/mol) 650,012 635,940 621,216 623,703 Polydispersity Ip 5.014 5.618 5.203 4.613 E-SBR A and E-SBR B of the invention each exhibit a microstructure similar to that of 5 the "controls" E-SBR C and E-SBR D. It may also be concluded that E-SBR A and E-SBR B of the invention each have: - a fatty acid content (essentially consisting of stearic and palmitic acid) which is less than one tenth of that of the "controls" E-SBR C and E-SBR D, and - a soap content which is reduced by approx. 25% relative to that of the "controls" E 10 SBR C and E-SBR D.

11 Analysis was performed to identify the compounds present in the ethereal phase of each of these elastomers, these phases being obtained from a dry toluene/ethanol extract. Mass spectrometry was used for this purpose. 5 1) Analytical method: The dry extracts corresponding to the ethereal phases were redissolved in dichloromethane, then esterified with tetramethylammonium hydroxide. The resultant solutions were analysed by combining gas phase chromatography and 10 mass spectrometry techniques. a) Mass spectrometry: The following equipment and parameters were used: 15 - "HP MSD5973" spectrometer; - electron impact ionisation; - a scanned range of masses: 33 to 550 amu; - 1300 V multiplier. 20 b) Gas phase chromatography: The following equipment and parameters were used: - "HP 6890" chromatograph; - "INNOWAX" column characterised by a length of 30 m, an internal diameter of 25 0.25 mm, a phase consisting of polyethylene glycol and a film thickness of 0.15 pm; - carrier gas consisting of helium; - "split" injection; - injector temperature of 250'C; 12 - the following temperature programme: TI = 50*C D= 2min P1 = 15 0 C/min. 5 T2=250'C Interface temperature = 280'C. 2) Results: 10 The principal products identified, grouped together under the headword "emulsifier", are as follows: - For E-SBR A 15 TMQ monomer (polymerised 2,2,4-trimethyl-1,2-dihydroquinoline) 6PPD (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine) oleic acid. - For E-SBR B: 20 TMQ monomer 6PPD oleic acid. 25 - For E-SBR C: palmitic acid 6PPD stearic acid 30 oleic acid.

13 - For E-SBR D: myristic acid (14 carbon atoms) palmitic acid 5 6PPD stearic acid oleic acid. 10 It may be concluded from these analyses that the copolymers E-SBR A and E-SBR B according to the invention comprise oleic acid, but neither palmitic nor stearic acid, and they comprise TMQ monomer, unlike the "control" copolymers E-SBR C and E-SBR D.

14 II. Use of "control" elastomers and elastomers according to the invention E-SBR A and E-SBR B in a tread filled with carbon black: Testing was performed on: 5 - a tread composition according to the invention which comprises a blend of said elastomers E-SBR A and E-SBR B in comparison with: - a first "control" tread composition o comprising a blend of S-SBR prepared in solution and a high-cis polybutadiene (BR), and - a second "control" tread composition © comprising a blend of said "control" 10 emulsion elastomers E-SBR C and E-SBR D. More specifically, said polybutadiene is characterised by a cis-1,4 linkage content of approx. 93% and is obtained, for example, by the process described in French Patent Specification FR-A-i 436 607. The essential characteristics of said S-SBRs are as follows: 15 - 1,2 content (%) 58 - styrene content (%) 25 - trans content (%) 23 - extender oil (phr) 37.5 20 - Tg (C) -29 - Mooney ML(1+4) 54. 1) Formulation and properties of the rubber compositions: 25 Table II below shows, on the one hand, the formulation of each of the above-stated rubber compositions and, on the other, processing properties (in the unvulcanised state) and physical properties (in the vulcanised state) obtained for these same compositions.

15 Table II: Comp. of the Comp. D Comp. M invention FORMULATION (in phr) E-SBR A 68.75 E-SBR B 68.75 E-SBR C 68.75 E-SBR D 68.75 S-SBR 110 BR 20 Black N234 85 85 85 High viscosity 7.5 5 aromatic oil ZnO/stearic acid 2.5/0.5 2.5/0.5 2.5/0.5 6PPD/ozone wax 1.9/1.5 1.9/1.5 1.9/1.5 Sulphur/CBS 1.45/1.45 1.4/1.4 1.45/1.45 PROPERTIES (measured on profiles) ML(1+4) 115 117 90 Shore A 64.5 64.4 64.5 M100 1.54 1.55 1.5 Table II shows that the elastomers E-SBR A and E-SBR B impart processing 5 properties to the rubber composition according to the invention which are similar to those imparted by the S-SBR to the corresponding "control" composition o. Table II also shows that the rigidity in the vuleanised state of the composition according to the invention is similar to that of the "control" composition Q based on conventional emulsion SBRs. 10 Table III below shows the viscoelastic properties of these rubber compositions. Table III: Comp. of the Comp. D Comp. Q invention G* 2.80 2.70 2.70 at 10% and at 40 0 C tg6 at 7% 0.440 0.460 0.470 and at 40'C I 16 Table III shows that the elastomers E-SBR A and E-SBR B impart reduced hysteresis to the rubber composition according to the invention relative to that imparted by "control" composition © based on conventional emulsion SBRs (tgS at 7% deformation). 5 2) Rolling wear resistance tests for treads consisting of these rubber compositions: Wear resistance testing was performed on a model "MXT" tyre of dimensions 175/70 R14 with a tread according to the invention and on tyres of the same dimensions and model 10 comprising treads corresponding to said "controls" © and D. Wear resistance values were determined by means of a relative wear index calculated on the basis of remaining rubber depth after driving on a winding road circuit until wear reaches the wear indicators arranged in the grooves of the treads. This relative wear index was obtained by comparing the remaining rubber depths of 15 the E-SBR-based treads (i.e. treads Q and treads according to the invention) with the remaining rubber depths of the S-SBR-based treads (i.e. treads ©), a reference baseline of 100 being assigned to this latter remaining rubber depth. A relative wear index of greater than this baseline of 100 indicates improved wear resistance relative to said tread o. 20 The wear results are set out in Table IV below. Table IV: Tread of the Tread © Tread © invention (reference) Relative wear index 117 100 98 25 In the light of this Table, it would seem that the wear resistance of the tread according to the invention is 19% better than that of a tread comprising E-SBRs having an emulsifier content of greater than 4 phr, such as tread (.

It will be noted that this improvement in wear resistance is essentially due to the reduced emulsifier content in the tread composition according to the invention, relative to composition Q. 5 It may consequently be concluded from these examples that use according to the invention of an emulsion SBR having an emulsifier content of between 1 and 3.5 phr in a tyre tread composition brings about a substantial improvement in the wear resistance of said composition and reduces the hysteresis losses thereof, relative to identical use of a conventional emulsion SBR having an emulsifier content of greater than 4 phr without there 10 being any degradation of other properties in the vulcanised state.